Hello! I plan to have tetras in my 46g aquarium, but I learned that tetras prefer soft water. The water in my area is hard (not sure what the exact pH is though) Therefore, how do I properly acclimate a fish that prefers soft water into hard water?

pH isn't a true indicator of whether water is hard or soft. GH or General Hardness is the indicator of whether water is hard or soft. It is common for water with a high pH to be hard, but yet it's possible to have soft water with a high pH too. You can either buy three test kits, pH, GH, and KH and test your water yourself...or you can have your Local Fish Store test a sample of your water for you...or you can obtain a water report from your local water utility unless you have well water. Anyway, the point is that in order to advise you properly we need to know what your pH, GH, and KH are...for most fish they will adapt quickly to water that is outside their "ideal" pH and water hardness parameters...what is more important is to provide clean water that has stable conditions and parameters.

We have a 10-gallon planted freshwater tank with one dwarf neon gourami, and a few pond snails (the ones d. gourami hasn't eaten yet). The pH is ~7.3. I'd like to stabilize the pH to 6.8, and I want proper water hardness to provide a healthy environment for a couple of amano shrimp that we plan to add to our tank in the near future.

We have low alkaline water that comes out of the tap (<40 ppm), so the buffering capacity ain't all that good. A dude at a LFS suggests that I place some peat-moss in our filter (lower the pH), and add some crushed coral to increase hardness and modify the buffering capacity.

How much crushed coral per gallon do I need to add to raise hardness by 1 ppm, 10 ppm, ?? ppm?

How large a piece (slice?) of peat moss do I need to put in my filter?

Is it a good idea to supplement our aquatic environment with peat moss and crushed coral?

Crushed coral will add calcium to harden e water, but also raise the pH.
You probably want to buy a non-carbonate source of calcium if you want acidic and hard water. HTH_________________"Just don't look in my fish room honey... it's just better for all of us. "

calcium and magnesium (GH) can affect fish in a few ways. A couple of important ways are:
1. fish adapted to hard water absorb these minerals from the water column to help build and run their bodies.
2. egg membranes harden due to calcium in the water column. Failure of the eggs to toughen up can be pollution killing the developing fry. Too hard of water for a species and the eggs can harden prior to fertilization.

Are you sure the hardness in he tank is that high? Is this from evaporation? shells or coral? rocks?
Something is adding to the GH...
good news, if you aren't breeding your fish, it is mostly harmless to the average aquarium raised stock. If the fish you listed are tank raised stock or unknown from a LFS, then they should be fine.

HTH_________________"Just don't look in my fish room honey... it's just better for all of us. "